Other Assets and Liabilities - Additional Information (Detail) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021 USD ($) Loans	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) Loans	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
CECL reserve	$ 130,388	$ 128,945	$ 172,100	$ 177,026	$ 178,050	$ 112,694	$ 130,388	$ 177,026	$ 173,549		$ 0
Increase (Decrease) in current expected credit loss reserve	158	(767)	(834)	(2,086)	(1,003)	4,677
Current expected credit loss reserve	$ 280	122	889	2,033	4,119	5,122	$ 280	2,033	1,723		0
Number of loans receivable | Loans	108						108
Credit Loss Reserve [Member]
CECL reserve	$ 280,000			2,000			$ 280,000	2,000
Increase (Decrease) in current expected credit loss reserve	(158,000)			(2,100)			(1,400)	1,600
Unfunded Loan Commitments [Member]
CECL reserve	5,203	$ 4,037	$ 11,021	12,057	$ 15,002	$ 22,536	5,203	12,057	$ 10,031		0
Increase (Decrease) in current expected credit loss reserve	$ (1,200)			$ (2,900)			$ (4,800)	8,800
Number of loans receivable | Loans	108						108
Unfunded loan commitments	$ 4,200,000						$ 4,200,000
Accounting Standards Update 2016-13 [Member]
CECL reserve										$ 13,942
Current expected credit loss reserve										445,000	$ 445
Accounting Standards Update 2016-13 [Member] | Unfunded Loan Commitments [Member]
CECL reserve	$ 3,300						3,300			$ 3,263
Increase (Decrease) in current expected credit loss reserve							$ 5,200	$ 12,100